1. Field of the Invention
The present invention relates to substrate processing and, more particularly, to moving substrates.
2. Prior Art
U.S. Pat. No. 4,715,921 discloses a substrate processing apparatus. U.S. Pat. No. 4,213,698 discloses a holder for vacuum holding a substrate. Gravity only substrate holders are also known in the art. Typical vacuum gripping substrate holders have a reputation for dropping wafers when they are moved about one percent of the time due to a failure of vacuum holding to initially occur. This often requires the substrate processing apparatus to be stopped and requires operator interaction. For a 100 wafer per hour throughput apparatus, although only one wafer per hour is not properly vacuum held, stopping the apparatus and operator interaction could reduce the throughput about 25% to only 75 wafers per hour. However, vacuum holders are much faster than gravity only holders because gravity only holders can only be accelerated and decelerated at a limit of about 0.1 g to prevent scratching of the wafer.